Alloy heat treatment



United States Patent 3,536,542 ALLOY HEAT TREATMENT Hugh .1. Murphy, Schenectady, and Chester T. Sims,

Ballston Lake, N.Y., assignors to--General Electric Company, a corporation of New York ICC able that the alloys used in the gas turbine hot stages be able to withstand mechanical stress over long periods of time. It is accordingly a primary object of this invention to produce such alloys of the above nickel-base type which not only are resistant to surface degradation at No Drawing. Filed May 31, 1968, Ser. No. 733,259 5 elevated temperatures, but which are further characterized Int. Cl. C21d 1/00 by much improved life at such temperatures under the 143-162 5 C ai stress-rupture conditions to which they are subjected in actual use.

1 Briefly, according to the present invention, alloys of ABSTRACT OF THE DISCLOSURE the nickel-base type set forth are subjected to a heat treat- The elevated temperature stress-rupture life of nickelment which compl'isoa treating at a temperature of about base alloys containing as major constituents nickel, cobalt, 2100 to 2200 tot about tWo to eight hours, P chromium, aluminum, titanium, and molybdenum, which y about 2125 about 4 hours followed y fast are also characterized by good oxidation and hot corro- Cooling, treating at a temperature of about 1975 sion resistance at elevated temperature, is improved by a about two to eight hours, Preferably four hours followed special heat treatment. The heat treating method comy fast cooling, treating at a temperature of about 17003 prises treating the alloy to a temperature of from about from about 12 hours to 48 hours, Preferably about 2100 F. to 2200 F. for from aboutZto 8 hours followed 24 hours followed y fast cooling, and finally treating by fast cooling, treating at a temperature of about 1975 at a temperature of about 1400 R for about 3 to 25 F. for from about 2 to 8 hours, fast cooling, treating said hours, Preferably 15 hours followed y fast Cooling- It alloy at a temperature of about 1700 F, for fro ab t has been found that this new heat treatment provides a 12 t b t 48 hours f ll d b fast li d treatsignificant increase in rupture life at elevated temperatures ing said alloy at a temperature of about 1400 f r without degradation of the other properties of the alloys f b t 3 t 25 hours foll wed b f t li 25 such as resistance to surface deterioration, yield strength,

ductility as represented by elongation and reduction in area, and the like.

Thi invention relates to a new d f l h treat- Those features of the invention which are believed to ment for alloys. More particularly, it relates to a new heat be novel are set forth With particularity in the Claims P- treatment for nickel-base, high temperature oxidationpended hofoto- The invention Will, however, be better and corrosion-resistant alloys which substantially imunderstood and further. advantages and j a o proves their stress-rupture life at elevated temperatures. appreclated from a conslderatwn of the followmg p- The use of nickel base alloys in equipment such as gas tionturbines which operate at elevated temperatures is well Amon the nickel-base alloys Which are considered to known. These materials are designed particularly to withbe particularly suitable to the new heat treatment are stand extreme stress for long periods of time, as well as those shown in Table I below.

TABLE I Composition, wt. percent Alloy Ni Co Cr Al Ti Mo B Zr 0 Prior art heat treatment New preferred heat treatment A) Specific Bal..- 15.0 14.6 4.3 3.35 4.2 0.016 0.04 0.07 (1) 2,125 F., 2hrs., furnace (l) 2,125 F., 4 hrs., fast cool. max. cool to 1,975 F. at 100 F./ (2) 1,075" F., 4 hrs., fast cool.

hr., air cool to room temper- (3) l,700 F., 24 hrs., fast cool. ature. (4) 1,400 F., 16 hrs., fast cool. (2) 1, 00 F., 16 hrs., cool to room temperature. Rango 13:11.-. 14.25 14.00 4.00 3.00 3.90 0.012 0.04 0. 05

15.75 15.25 4.60 3 70 4.50 0 020 max. 0. 00 (B) Specific Bal 18.5 15.0 4.3 3.5 5.1 0.010 0. 04 0.10 (1) 2,150 F.,4hrs., air eool 1) 2,125 F.,4hrs., fast 0001. max. (2) l,975 F., 4 hrs., air cool. (2) 1,975 F., 4 hrs., fast cool.

(3) 1,550 F., 24 hrs., air cool. (3) 1,700 F., 24 l11's., fast cool. (4) 1,400 F., 16 hrs., air cool. (4) 1,400 F., 16 hrs., fast cool. Range B211-" 14. 00 13.00 3.75 2.75 4.50 0.01 0.15

20.00 17.00 5.00 4.00 0.00 0.05 max.

2 Also, Fe, 4.0 max.

resist oxidation, sulfidation and other types of surface attack at such elevated temperatures.

Intensive work in the formulation of such alloys has resulted in compositions which are admirably suited for operation in corrosive atmospheres, such as those produced in a gas turbine, combined with a reasonably long life under such conditions before failure under mechanical stress. However, it is desired to develop machines such as gas turbines which operate at higher and higher temperatures and for longer times, since such characteristics increase the efiiciency and the economy of the device. For instance, an increase in operating temperature from about 1500 F. to about 1600 F. in a gas turbine represents an increase in power output of about 14% and an increase in efficiency of about 1 to 5%. Further, it is advantageous if a machine can operate for 40,000 hours instead of 20,000 hours.

Thus, it will at once become apparent that it is desir- It has been found that the heat treatment of this invention, while similar in some respects to prior art heat treatment, departs significantly from such prior art treatments particularly in the treatment at about 1700 F. which produces the improved mechanical qualities found in the heat treated material. The subject of such alloys and their compositions and characteristics are set forth in various publications including Journal of Metals, October 1966, pages 11194130.

According to this invention, the alloy is first heated at a temperature of from about 2100 to 2250 F. for from about 2 to 8 hours, and preferably at 2125 F. for about 4 hours and fast cooled as by a gasquench, such as by introducing cold inert gas into the furnace or by Withdrawing the alloy into the open air. The purpose of this heat treatment step is to solutionize the alloys. Temperatures above about 2250 F. would tend to cause incipient melting of the alloy and defeat the purpose of the heat treating step. During this treatment the major strengthening phase, [Ni (Al,Ti)] and M C carbides, (M=a metal atom such as chromium or molybdenum, so that. M C often has the composition Cr Mo C are placed in solution leaving only the continuous alloy matrix, and MC carbides (where a composition such as (Ti,Mo)C is often experienced) as such. A temperature of the least 2100 F. must be used because at lower temperatures certain of these phases will tend to percipitate in an uncontrolled fashion, and, as pointed out above, temperatures above about 2250 F. will produce incipient melting of the alloy. Treatment times of less than about 2 hours can not be expected to produce full solution, and treatment for more than about 8 hours for 2125 F. would be uneconomical. After this treatment the alloy is fast cooled as above.

Secondly, the alloy is held at a temperature of about 1975 F. for from about 2 to 8 hours and preferably for about 4 hours. The object of this treatment is to initiate precipitation of the 'y strengthening phase, which is distributed generally as a finely divided particulate precipitate. This initiation of precipitation at this temperature results in the development of optimum size 7' particles upon subsequent aging, which contributes to the achievement of maximum strengthening. Treatment in this temperature regime for less than about 2 hours results in inadequate precipitation, and again more than about 8 hours treatment would be unnecessary and uneconomical. Once again, the alloy at the end of this treatment is fast cooled as described above.

The third step in the present heat treatment is critical, is the essence of this discover, and is the major single variant from prior art practice. This treatment, first of all, causes the 7' particles precipitated as described above to grow to an approximate optimum size for the generation of maximum strengthening. Secondly, and of most importance, this treatment precipitates M C carbides at grain boundaries through a solid state reaction which is believed to be as follows:

As pointed out above, the M C compound percipitates as discrete carbide particles which are of optimum size at the grain boundaries, strengthening these boundaries and locking or fixing them to prevent or delay undesirable slippage. An important part of this heat treatment is believed to reside in the fact that the 'y' degeneration product develops as a continuous envelope around the M C carbides precipitated at the grain boundaries. This englovement or surrounding of M C particles by prevents subsequent precipitation of deleterious cell-like M C at the grain boundaries during actual service of the alloy, an experience found in the past which can cause failure of parts in service. This englovement also acts as a strong ductile cushion inhibiting the onset of alloy fracture and promoting long rupture life. While the above reaction can also occur during the service life of the alloy in the event the alloy is exposed to this temperature range, the uniqueness of the present heat treatment lies in the fact that this treatment forces the reaction toward completion prior to service so that the alloy initially is in the safest and strongest possible condition. A similar effect can be obtained by use of shorter hours of reaction time, down to about 12 to 16 hours; longer reaction times of the order of 36 to 48 hours, while acceptacle, would be uneconomic. Nonetheless, 24 hours reaction time appears to be optimum.

In the fourth and last step of the present heat treatment, which is again traditional in such alloys of the present type, the alloy is heated at 1400 F. for 16 hours and fast cooled as above. This final treatment causes a further precipitation of the 7' phase and may also bring a small additional amount of M out of solution. As a general proposition it tends to stabilize the alloy as a whole. Alternatively, the time of treatment can be varied from about 8 to 24 hours.

In Table H below are shown the stress-rupture lives of various numbered heats of specific alloy A above which were subjected to the temperatures and stresses indicated,

such. alloys having been heat treated as shown in Table I above for alloy A, the time and temperature of treatments being those indicated above as preferred. This improvement is expressed as the ratio of the stress rupture life for the new treatment as opposed to that for the old treatment.

TABLE 11 Improvement over Temp. stress prior art heat Heat F.) (k.s.1.) Life (hrs.) treatment (times) From the above data it will at once be apparent that the present heat treatment affords a tremendous advantage over that of the prior art, the life as shown in the table being improved from about 1.7 to about 4.6 times.

Thus, there is provided by the present invention a heat treatment for high-temperature-resistant, nickel-base alloys of the type described which substantially improves the stress-rupture life of such alloys.

What We claim as new and desire to secure by Letters Patent of the United States is:

1. A method of heat treating a nickel-base alloy containing as major essential constituents in percent by weight, cobalt about 14 to 20, chromium about 13 to 17, aluminum about 3 to 5, titanium about 2.5 to 4, molybdenum about 3.5 to 6, boron about 0.01 to 0.05, zirconium about 0.04 max, carbon about 0.03 to 0.15, with the balance nickel, which consists essentially of (1) treating the alloy at a temperature of from about 2100 F. to 2200 F. for from about 2 to 8 hours followed by fast cooling, (2) treating at a temperature of about 1975 F. for from about 2 to 8 hours, fast cooling, (3) treating said alloy at a temperature of about 1700 F. for from about 12 to about 48 hours, fast cooling, and (4) treating said alloy at a temperature of about 1400 F. for from about 8 to 25 hours and fast cooling.

2. A method as in claim 1 where (1) the time of heat treatment at about 2100 F. to 2200 F. is about 4 hours, (2) the time of heat treatment at about 1975" F. is about 4 hours, (3) the time of heat treatment at about 1700 F. is about 24 hours, and (4) the time of heat treatment at about 1400 F. is about 16 hours.

3. A method as in claim 1 wherein said alloy contains as major essential constituents in percent by weight, cobalt about 14.25 to 15.75, chromium about 14.00 to 15.25, aluminum about 4.00 to 4.60, titanium about 3.00 to 3.70, molybdenum about 3.90 to 4.50, boron about 0.012 to 0.020, zirconium about 0.04 max, carbon about 0.05 to 0.09, with the remainder nickel.

4. A method as in claim 1 wherein said alloy contains as major essential constituents in percent by weight, cobalt about 15.0, chromium about 14.6, aluminum about 4.3, titanium about 3.35, molybdenum about 4.2, boron about 0.016, zirconium about 0.04 max, carbon about 0.07, with the balance nickel.

5. A method as in claim 4 wherein said alloy is heat treated at about 2125 F. for about 4 hours, fast cooled, (2) treated at a temperature of about 1975" F. for about 4 hours, fast cooled, (3) treated at a temperature of about 1700 F. for about 24 hours, fast cooled, and (4) treated at a temperature of about 1400 F. for about 16 hours and fast cooled.

6 3,147,155 9/1964 Lamb 148-162 3,415,641 12/1968 Ross 148162 X OTHER REFERENCES References Cited 5 Journal of Metals, October 1966, pp. 1119-1130.

UNITED STATES PATENTS CHARLES N. LOVELL, Primary Examiner 10/1951 Bieber et a1. 148-162 10/1956 Betteridge et a1. 148-162 X 8/1964 Hignett et al 148162 10 75171; 148-32.5 8/1964 Bird et a1. 148162 

